Hui Pan, Xinshuang Li, Yichao Shen, Xiang Wu, Feng Ju, Yuzhe Li, Gaosheng Wu, Bo Ai,Baoyun Xu, Hao Ling,*

1 School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

2 Shanghai Research Institute of Chemical Industry Co., Ltd., Shanghai 200062, China

Keywords:Hydrocracking tail oil Lubricant-type vacuum distillation process Thermal coupling intensified technology Dividing-wall column

ABSTRACT Dividing-wall columns (DWCs) are widely used in the separation of ternary mixtures, but rarely seen in the separation of petroleum fractions.This work develops two novel and energy-efficient designs of lubricant-type vacuum distillation process (LVDP) for the separation of hydroisomerization fractions(HIF) of a hydrocracking tail oil (HTO).First, the HTO hydroisomerization reaction is investigated in an experimental fixed-bed reactor to achieve the optimum liquid HIF by analyzing the impact of the operating conditions.A LVDP used for HIF separation is proposed and optimized.Subsequently, two thermal coupling intensified technologies,including side-stream(SC)and dividing-wall column(DWC),are combined with the LVDP to develop side-stream vacuum distillation process(SC-LVDP)and dividing-wall column vacuum distillation process (DWC-LVDP).The performance of LVDP, SC-LVDP, and DWC-LVDP are evaluated in terms of energy consumption, capital cost, total annual cost, product yields, and stripping steam consumption.The results demonstrates that the intensified processes, SC-LVDP and DWC-LVDP significantly decreases the energy consumption and capital cost compared with LVDP.DWC-LVDP further decreases in capital cost due to the removal of the side stripper and narrows the overlap between the third lube oils and fourth lube oils.This study attempts to combine DWC structure into the separation of petroleum fractions, and the proposed approach and the results presented provide an incentive for the industrial implementation of high-quality utilization of HTO through intensified LVDP.

1.Introduction

Hydrocracking technology is extensively applied in modern petroleum refining industry in that it converts heavy oil fractions into lighter fractions in meeting the need for clean fuels, effective feedstocks for FCC and petrochemical operations, and high quality lubricating oils.However,unconverted products of the hydrocracking reaction called hydrocracking tail oil (HTO) may be up to 10%-40% of the original material [1-3].Previously, HTO was used as feedstock blending oil for FCC or ethylene cracking,which reduces the use value of HTO and increases the energy consumption of the entire process [4,5].Recently, the resource utilization of HTO that produces lube base oil received continuous attention, owing to the growing demand for non-polluting and high-quality lubricating oil [6-9].To produce high-quality lube base oil, HTO is always treated using two steps, which include a reaction treatment (such as hydroisomerization, hydrodewaxing, or nonhydrodewaxing reaction)and a separation treatment(such as a lubricant-type vacuum distillation process (LVDP)).

Some studies have been reported for the reaction treatment of HTO in terms of the performance of catalysts [10-14].Sivasankeret al.[10]investigated the influence of crystallite size, silicaalumina ratio and isomorphous substitution by Fe3+ions on the shape-selective cracking characteristics of ZSM-5 zeolite in the hydrodewaxing process.The results showed that changes in the Si/Al ratio did not affect the catalyst performance and a decrease in crystallite size enhanced catalyst activity and selectivity, but reduced the deactivation of ZSM-5 zeolites.The HZSM-5/Al2O3molecular sieve catalyst was modified using phosphoric acid,tetraeth-oxysilane, tetrabutyl titanate, and boric acid, and their performances were then evaluated in the nonhydrodewaxing reaction of HTO.The results showed that the Si/HZSM-5/Al2O3catalyst has better catalytic dewaxing effect and stability than the other modified catalysts,owing to its stronger acid sites and larger specific surface areas [11].However, HZSM-5/Al2O3catalyst is disadvantageous for long chainn-alkane to generate carbonium ion.Hence, it is suggested that chain transfer agent is added to HTO as a raw material to accelerate carbocation generation, such as low-carbon alcohol.Tert-butyl alcohol exerts significant effect on the improvement of carbonium ion formation in the reaction of HTO, owing to its acceleration of carbocations generation [12].Additionally, the impact of wax content in hydrocracker unconverted oil on viscosity index and yield of lubricant base oil was studied[13].The results showed that in order to meet the low pour point requirement of products, an increase in the wax content of the unconverted oil would result in an increase in the hydroisomerization dewaxing reaction temperature.It is noted that the aforementioned studies have focused on the reaction treatment of HTO instead of the separation of its reaction liquid products.

The separation of the reaction liquid products is commonly achieved through a LVDP, which is complex and high in energy consumption.Hence, improving the energy efficiency of fraction distillation like the LVDP has become a research area of interest[15-17].Thermal coupling intensified technologies (such as dividing-wall columns, heat pumps, and side-stream distillation,etc.)have been proposed to enhance the energy efficiency of distillation systems [18-37].DWC technology, which is capable of separating ternary or quaternary mixtures in one vessel, has been studied extensively in terms of optimization design and dynamic controllability [18-28].However, few studies have focused on the separation of petroleum fractions using DWC technology[29-31].For example, a DWC structure was proposed to obtain benzene and toluene-rich fractions from a 15-component aromatics-rich mixture in a petroleum refinery [29].In the modelling,Vmindiagram method was used to achieve reliable initial parameters for the rigorous model.The numerical results illustrated that the DWC structure decreased by approximately 43%in energy cost compared with conventional direct distillation sequences in two-column configuration.The separation of a typical mixture of refinery cuts was also investigated in a train of thermally coupled distillation columns within DWC technology.The train of DWC structure was optimized by coupling a multiobjective genetic algorithm with Aspen Plus simulator.It was observed that the optimized DWC can maintain minimum energy operation even at some non-optimal operating conditions[30].Additionally, sidestream thermal coupling intensified technology also has been combined into multicomponent distillation systems[32-37].Emtir and Etoumi[32]introduced a sidedraw stream at peak point composition of middle component in the first column to transfer into the second column for the separation of alcoholic and aromatic mixture systems.Two improved direct and indirect configurations were proposed, which significantly decreased the TAC compared with direct and indirect traditional configurations.Additionally,the improved indirect configurations were superior to the improved direct sequences within the decrease in the molar fraction of middle component in the feed.This is because the reduction of remixing effect and the improvement of thermodynamic performance in the configurations.To prevent the accumulation of acetaldehyde in the system,a side withdrawal was also introduced into pressure swing distillation for the separation of homogeneous azeotropic methyl acetate/methanol mixture with a small amount of acetaldehyde[33].Various side-stream configurations were proposed and compared for the separation of benzene-toluene-xylene mixture.The optimization results illustrated that the remixing effect and feed mismatch can be potentially reduced with the aid of the side stream transfer streams[34].They further implemented DWC technology into a lubricant type vacuum distillation unit for crude distillation using a decomposition method derived from the procedure proposed by Errico [38]and Rong [39].As a result, the lube base oil product yield was improved and the boiling point range of the fourth lube cuts was reduced[35].Thus,thermal coupling intensified technologies potentially provide solutions for the energy-efficient improvement of lubricant-type vacuum distillation processes (LVDPs) in the reutilization of HTO.

The aim of this work is to develop novel and energy-efficient configurations of LVDPs for hydrocracking tail oil (HTO).First, the hydroisomerization reaction for HTO is investigated in an experimental fixed-bed reactor within a commercial catalyst, and the effect of the volume space velocity, temperature and hydrogenoil ratio are considered.Subsequently, an optimized LVDP including a prefractionator and vacuum distillation column (VDC), is developed to separate HIF.Finally, two thermal coupling intensified technologies, side-stream and DWC, are combined with the optimized LVDP to develop two novel configurations.The capital cost, total annual cost (TAC), product yields and stripping steam consumption are analyzed to assess the novel configurations of the LVDP.

The high-quality utilization of HTO produces lube base oils through reaction and separation treatments.In contrast to other HTO studies that only focus on reaction systems, this study comprehensively investigates the reaction system to further focus on the optimization of LVDP.Two thermal coupling intensified technologies including SC and DWC are first proposed for the separation of HIF in the optimized LVDP to produce lube base oils.These two thermal coupling intensified technologies can be easily implemented for LVDP in industry.DWCs are widely used in the separation of ternary mixtures, but rarely seen in the separation of petroleum fractions.This study attempts to combine DWC structure into the separation of petroleum fractions, and increases the incentive for the industrial implementation of intensified LVDP,owing to its significant potential for energy consumption, capital investment savings and product distribution changes.Based on this comprehensive trial,researchers could go further and conduct more in-depth studies for lube base oils production from HTO using thermal coupling intensified technology, which provides a potential general solution for the industrial implementation of high-quality utilization of HTO through intensified LVDP.

2.Hydroisomerization

2.1.Experimental setup

Fig.1 shows the hydroisomerization process,which mainly consists of a thermostatic water bath,a fixed-bed reactor,a buffer tank and a liquid product tank.The temperature of thermostatic water bath was maintained at 70 °C.The inner diameter of the fixedbed reactor was 10 mm.The upper and lower sections of the fixed-bed reactor were filled with quartz sands.The middle section of the reactor,within a height of 170 mm,was loaded with 12 ml of the catalyst mixture,which consisted of 12 ml of the catalysts and an equal volume of pretreated quartz sands.First,the bed temperature was increased at 10°C·min-1to 360°C and was kept for half an hour under nitrogen and oxygen atmosphere.Then it was increased at 10°C·min-1to 460°C and was kept for an hour for catalyst regeneration at normal pressure.Catalyst reduction was achieved under a hydrogen atmosphere (at 320 °C and 8.5 MPa pressure) with a H2/catalyst ratio (VH2/Vcatalyst) of 1200:1 for 8 h before the experiment.

2.2.Effect of operating conditions

Fig.1. Schematic diagram of the hydroisomerization process:1-Hydrogen;2-Nitrogen and oxygen;3-Raw material tank;4-Thermostatic water bath;5-Gas flow controller;6-By-pass valve; 7-Advection pump; 8-Check valve; 9-PC control; 10-Small fixed-bed reactor; 11-Relief valve; 12-Pressure balance valve; 13-Buffer tank; 14-Product tank;15-Vent valve; 16-Gulp valve.

The reaction pressure was set to 8.5 MPa and is not discussed in this study.The effect of the volume space velocity (SV), temperature(T)and hydrogen-oil ratio(VH2/Voil)on the pour point,viscosity index(VI),and yield of HIF was investigated,and the results are presented in Table 1.Furthermore, the hydrocarbon structure types of the saturated portion separated from the HTO and HIF were analyzed, and an overall flowchart of the hydrocarbon compositional analysis is illustrated in Fig.2.The volume SV significantly affects the capacity of the fixed-bed reactor and the depth of the isomerization reaction.As shown in Table 1, an increase in the volume space velocity leads to an increase in the pour point and viscosity index of HIF and the liquid yield.In Fig.2(a),it is also observed that an increase in the volume space velocity decreases the alkane compounds, owing to the shallowness of isomerization and cracking reaction depth, which are the main reason for the increment in the VI and the liquid yield.Additionally,Fig.2 reveals that a small increase in low molecular naphthene (2-ring naphthene, 3-ring naphthene, and 4-ring naphthene) in HIF, compared with that in the HTO feed,was at the expense of 5-ring naphthene.The comparison of liquid product properties for the three reaction temperatures listed in Table 1 demonstrates that as the reaction temperature increased from 320 °C to 360 °C, the pour point and liquid yield decreased by 36°C and 7%,respectively.This is because isomerization and cracking reactions are enhanced simultaneously.However, an excessively high reaction temperature may weaken the selectivity of the isomerization reaction; hence,liquid product VI first increases then decreases,and the peak appears at a reaction temperature of 340 °C.The total amount of alkane, 1-ring naphthene, and 2-ring naphthene at 340 °C was higher than that of other reaction temperatures (Fig.2(b)), which also indicates that the ring-opening reaction for polycyoalkane occurred vigorously at a reaction temperature of 340 °C.From Fig.2(c), it is observed that the increase of 3-ring naphthene and 4-ring naphthene results from an improvement of the hydrogen-oil ratio, whereas the total amount of alkane, 1-ring naphthene, and 2-ring naphthene reduces; this is particularly prevalent for the amount of alkane.This is because the high hydrogen-oil ratio strengthens the isomerization over cracking reaction so that long-chain alkanes or polycyoalkanes are difficult to crack or ring-open.Consequently, the increase in the high hydrogen-oil ratio leads to a clear increase in the yield and decrease of VI as shown in Table 1.Based on a comprehensive consideration of the impact of various operating conditions, the optimized reaction temperature was 340 °C, the volume SV was 1.25 h-1, and the hydrogen-oil ratio was 640.As a result,the viscosity index of the liquid product can reach 127, and the pour point can be reduced to -38 °C.

Table 1Effect of various operating conditions on properties of liquid HIF

Fig.2. Effect of various operating conditions on hydrocarbon structure type distributions of products: (a) volume space velocity; (b) temperature; (c) hydrogen-oil ratio.

3.Design of LVDP

The liquid HIF under the optimized operating conditions were collected and further analyzed using true boiling point distillation method(TBP).As a result,the cutting scheme determined that the 350-500°C fraction of HIF is cut into four lube base oils by taking 390°C,410°C,and 450°C as cutting points.150-350°C fraction is distillated as a light pseudo component to produce industry white oil,and the 500-540°C fraction is distillated as heavy pseudo component to return to the hydrocracking unit for further processing.Hence, an LVDP that includes a prefractionator and a VDC is proposed using Aspen Plus software (AspenTech) and is displayed in Fig.3.As shown in the figure, the light fraction (＜350 °C) is first removed from the top of the prefractionator and the base oil fraction at the bottom of the prefractionator is vaporized in a vacuum heat furnace and then introduced into the VDC for separation.As a result, four base oils (named as first lube cuts, second lube cuts,third lube cuts,and forth lube cuts)and heavy pseudo components(＞500 °C) are obtained at the side-stream and bottom of the lubricant-type VDC, respectively.

Fig.3. Simulation flowsheet of LVDP.

3.1.Simulation and optimization of the prefractionator

The flow rate of HIF is set as 50,000 kg·h-1at 200 °C, and the prefractionator and VDC is operated at 5 kPa.As the top pressure of the VDC is 5 kPa, the temperature of saturated steam is about 33 °C.Hence, the top temperature of the VDC is set as 50 °C to ensure that the steam will not liquefy, and the products can also be drawn completely from the sidestream.Rigorous simulations were performed with the Braun K-10 thermodynamic model through the PetroFrac modules in Aspen Plus.Moreover,the number of the stages and the location of the feed stage for the prefractionator are optimized to minimize the TAC.The optimized variables and the equations and corresponding parameters used for the TAC calculation are listed in Fig.S1(a)and Table S1,respectively,in Supporting Information.Fig.4 depicts the economic optimization of the prefractionator by the varying stages and feed stage locations.As shown in Fig.4(a), as the number of the stages increase, the TAC of the prefractionator first decrease and then increases, and the peak appears at 9 stages.The variation trend is consistent with that of the reboiler duty, which illustrates that the energy cost significantly affects the TAC of the prefractionator.Fig.4(b) shows the effect of the location of the feed stage on the TAC, which demonstrates that the rise of feed position results in the increase of the diameter of the prefractionator (ID) and the decrease of heat duty,hence the optimized feed stage is considered as stage 5 to minimize TAC.

3.2.Simulation and optimization of the vacuum distillation column

To simplify the design, the basic configuration of the VDC is depicted in Fig.5.As shown in Fig.5, the VDC possesses the four lube base oils and one heavy component.Generally, 5-7 theoretical stages are set between each side-stream;therefore,26 theoretical stages are set for the VDC in this study.Furthermore,three side strippers(named as S-1,S-2,and S-3)are added to the VDC and the side drawn stage are 7, 14, and 20, respectively.Notably, the TBP 95%points of the four lube base oils are controlled by manipulating the flow rate of the four lube base oils,while the TBP 5%points are controlled by manipulating that of the stripping stream.In addition, except for the pump-around circuit (P-1), two intermediate refluxs(P-2 and P-3)were added to the VDC and were further optimized in terms of the drawn/returned stages and heat duty.The drawn stage and heat duty of intermediate refluxs have significant impact on the hydraulics performance of gas-liquid flow in the column.Fig.6 presents the optimization of the drawn stage and the heat duty of P-2.It is clear that the improvement in the P-2 located stage results in an increase in the flow rate of the first lube cuts and the TBP 5% point of the second lube cuts (see Fig.6(a)).This is because the increase in the located stage of P-2 results in the decrease in the temperature of the extraction flow;hence,the temperature of return flow decrease.As a result, more heavy component in the gas flow condense, which improves the separation performance of fraction in the regions above the intermediate reflux (P-2).As the drawn stage of P-2 is selected as stage 6, the sum of flow rate of second lube cuts and third lube cuts reaches a maximum.Moreover, the increase of TBP 5% point of the second lube cuts leads to the decrease of the flow rate of Steam-2 to reduce energy consumption.Fig.6(b) indicates that the heat duty of P-2 has minimal impact on the flow rate, TBP 5% and TBP 50%points of the four side-stream products.However, the decrease in P-2 heat duty results more vapor entering into the top regions of the column, thereby increasing the P-1 heat duty.As a result, the reflux temperature decreases and reflux ratio increases, which improves the separation performance of fraction in the regions above the intermediate reflux(P-2).However,lower reflux temperature means lower temperature of cooling temperature.When the P-2 heat duty is lower than 1.4 MW, the reflux temperature is lower than 35 °C.Thus, a 1.45 MW P-2 heat duty is selected to maintain suitable reflux temperature and maximum sum of the flow rate of the first lube cuts and second lube cuts.Similarly,the optimal drawn stage for P-3 is 11, and the maximum TBP 5%point of the third lube cuts is achieved.In addition, the optimal heat duty of P-3 is 1.3 MW, and the sum flow rate of the second and third lube cuts reaches a maximum.

Fig.4. Optimization of the prefractionator: (a) number of stages; (b) feed stage locations.

Fig.5. Basic configuration of the VDC.

Fig.6. Optimization of the intermediate reflux P-2 in VDC: (a) drawn stage of P-2; (b) heat duty of P-2.

The final optimal LVDP including the prefractionator and VDC is illustrated in Fig.7.Table 2 presents the distillation data of four lube base oils in the LVDP.Apparently, the TBP 95%points of the four lube base oils fulfills the requirements.Furthermore, the TBP 50% points for any of the product oils are less than or equal to the TBP 5% points of its neighbor product oil.In addition, the overlap between the first and second lube cuts, second and third lube cuts, third and fourth lube cuts,fourth lube cuts, and heavy oils is 26.0 °C, 22.9 °C, 37.4 °C,and 48.4 °C, respectively.The largest overlap occurs between the fourth lube cuts and heavy oils, which results from low vapor and liquid capacities at the bottom of the VDC (see Fig.13(a)).

Table 2Distillation data of four lube base oils in LVDP process

Fig.7. Final optimal configuration diagram of LVDP.

Fig.8. Simulation flowsheet of SC-LVDP.

4.Novel Design of LVDP based on Thermal Coupling Intensified Technology

4.1.Side-stream coupling into LVDP

The side-stream thermal coupling intensified technology is introduced into the optimized LVDP process to extract a side liquid stream from a stage below the feed stage of the prefractionator into the VDC(see Fig.8).Notably,the feed position of the side liquid stream is located above the original feed stage of the VDC.The side-stream coupling LVDP (SC-LVDP) is easily implemented during practical process renovation because this technology only requires the addition of a pipeline and control valve between the prefractionator and LVDP,which represents remarkable applicability and implementation prospects in industrial applications.

The coupling of the side-stream into the LVDP results in three new design variables, including the flow rate of the side liquid stream from the prefractionator, the side liquid stream drawn stage in the prefractionator, and the side liquid stream feed stage in the VDC(Fig.S1(b))in Supporting Information).The three design variables are optimized to minimize the sum heat duty ofQRandQF, improving the quality of the lube cuts, or decreasing the amount of stripping steams.Table 3 depicts the effect of the side liquid stream drawn stage on the flow rate of the four lube base oils and stripping steams.It is observed that the side liquid stream drawn stage did not have significant impact on the flow rate of the four lube base oils.However, the flow rate of stripping steams increases with an increase in drawn position of the side liquid stream,owing to the decrease in the TBP 5%points of the four lube base oils.However, more vapor is required to maintain sufficientliquid phase to be withdrawn from the higher location in the prefractionator; thus, the reboiler heat duty of the prefractionatorQRimproves.As shown in Table 3,the vacuum heat furnace heat loadQFdecreases significantly, owing to lower flow rate of the base oil fraction compared with that in the LVDP.Stage 8 is selected as the side liquid stream drawn stage, and the energy consumption decreases by approximately 14.08%.Fig.9 presents the effect of the flow rate of the side liquid stream on that of the four lube base oils and stripping steams.It is observed that the flow rate of the four lube base oils and stripping steams are affected significantly by the flow rate of the side liquid stream.The increase in the flow rate of the side liquid stream leads to the increase in the flow rate of the second lube cuts, fourth lube cuts, heavy stream, and stripping steams and the decrease in the flow rate of third lube cuts.Notably, the flow rate of stripping steams is designed to be lower than 10%of the flow rate of the corresponding lube cuts to prevent the increase of the number of theoretical trays.However, as the flow rate of the side liquid stream is higher than 15,000 kg·h-1,the flow rate of lube cuts and stripping steams varies dramatically that the flow rates of Steam-2 and Steam-3 are higher than 12%of that of the second lube cuts and third lube cuts,respectively.Consequently, the optimal flow rate of the side liquid stream is 15,000 kg·h-1.The influence of the feed position of the side liquid stream in the VDC onQR,QF, the quality of the 3rd lube cuts, and the amount of stripping steams are shown in Table 4.It is observed that the feed stage of the side liquid stream has little effect onQRandQF, but affects greatly on the quality of the lube cuts and the amount of stripping steams.As shown in Table 4, when the feed position is higher than stage 15,the TBP 95%point of 3rd lube cuts deviates greatly from 450°C.Furthermore,the amount of stripping steams first decreases and then increases with decreasing the feed position of the side liquid stream in the VDC,and the peak appears at stage 15.Hence, the optimized feed stage of the side liquid stream into the VDC is stage 15.The final optimal SC-LVDP is illustrated in Fig.10.Compared with the configuration of LVDP in Fig.7,the introduction of side-stream results in the decrease in the flow rate of fraction entering into the VDC.Thus, the decreases of the total heat duty input for the VDC leads to the decrease in the heatduty of the pump-around circuit and two intermediate refluxs.Moreover, the vapor load of the VDC decreases results from the introduction of side-stream, which leads to the decrease in the diameter of the VDC.The distillation data of the four lube base oils in SC-LVDP demonstrated that the TBP 95% points of the four lube base oils fulfills the requirements (see Table S2 in Supplementary Material).Furthermore, the largest overlap still occurs between the fourth lube cuts and the heavy oils.

Fig.10. Final optimal configuration diagram of SC-LVDP.

Table 3Effect of side liquid stream drawn stage on flow rate of four lube base oils and stripping steams

Table 4Effect of feed stage of the side liquid stream into the VDC

Fig.9. Effect of flow rate of the side liquid stream on that of the four lube cuts and stripping steams.

4.2.DWC coupling into SC-LVDP

The DWC thermal coupling intensified technology is introduced into the VDC in SC-LVDP to reduce the number of side strippers,and the remixing effect in the VDC.The additional side stripper S-3 has a diameter within 1.09 m, which is relatively small compared with the main VDC;hence,this small-diameter side stripper can be retrofitted into the main column through DWC thermal coupling intensified technology.First, the VDC is decomposed into three simple columns (see Fig.11(a)), and the side stripper S-3 is then retrofitted into the main column within one middle partition through DWC technology in Fig.11(b).Similarly, five new design variables are introduced, including the coupled vapor flow rate(S-vapor), coupled liquid flow rate (S-liquid), specific location of the partition, flow rate, and draw stage of the forth lube cuts(Fig.S1(c) in Supporting Information).Notably, the TBP 95% point of the fourth lube cuts is controlled by its flow rate, whereas the TBP 5% point is controlled by S-vapor and S-liquid.To simplify the design, the location of the vessel is set between stage 21 and stage 25.Table 5 presents the optimized results by analyzing other four new design variables in DWC section.It is clear that with an increase in the S-vapor flow rate, the flow rates of the third lube cuts increases, while that of the fourth lube cuts decreases.When S-vapor flow rate is higher than 1100 kg·h-1, regulating S-liquid can no longer ensure that the TBP 5%point of the heavy component is higher than the TBP 50% point of the fourth lube cuts.This is because the stripping vapor brought heavy components upward.The optimal drawn stage and flow rate of the fourth lube cuts are set to stage 4 and 17,200.4 kg·h-1, respectively.The final optimal DWC-LVDP is illustrated in Fig.12, and the distillation data of the four lube base oils in DWC-LVDP are listed in Table S3 in Supporting Information.It is observed that the overlap between the third and the fourth lube cuts is 35.4 °C in DWC-LVDP, which is lower than that of LVDP and SC-LVDP.

Table 6Comparison of LVDP, SC-LVDP, and DWC-LVDP

Fig.11. Design of DWC-LVDP: (a) decomposition of VDC; (b) retrofitting of side stripper S-3 into the main column through DWC technology.

Table 5Optimization of dividing-wall section’s new design variables

4.3.Comparison of LVDP, SC-LVDP, and DWC-LVDP

Fig.12. Final optimal configuration diagram of DWC-LVDP.

Fig.13. Comparison of the vapor and liquid loads distributions in VDC of LVDP, SC-LVDP, and DWC-LVDP.

Fig.13 compares the distributions of the vapor and liquid loads in VDC of LVDP, SC-LVDP and DWC-LVDP.The distributions of the vapor and liquid loads in these three columns show similar overall trends that travel along zigzag paths.This is because internal reflux and side strippers are added; hence, the liquid loads decrease significantly at the drawn stage and increase at the feed stage of the internal reflux.In addition,the vapor and liquid loads first increase and then decrease in the three columns.The maximum vapor and liquid loads all occur at stage 11 for the three VDCs.However, the vapor and liquid loads of vacuum distillation column in SC-LVDP are lower than those of LVDP and DWC-LVDP.Owing to the introduction of the side-stream,the liquid loads at the feed stage of VDC increase and the vapor loads decrease significantly.A rapid decrease in the liquid loads in the DWC section of the VDC can be observed, which is owing to the withdrawal of the fourth lube cuts from fourth stage in DWC section (Stage 24 in the VDC).The sum of the energy consumption, capital cost, TAC, product yields,and stripping steam consumption for the three processes (LVDP,SC-LVDP, and DWC-LVDP) are further summarized and compared in Table 6.The results demonstrate that the energy consumption of the two novel designs, SC-LVDP and DWC-LVDP, is significantly lower than that of LVDP.Although SC-LVDP and DWC-LVDP exhibit the same energy consumption, the introduction of the partition wall significantly narrows the overlap between the third lube oils and fourth lube oils and also avoids the usage of Steam-4 in DWC-LVDP.The coupling of the side-stream in SC-LVDP effectively reduces the vapor and liquid loads of the VDC, thereby decreasing its capital cost.In addition,the capital cost of DWC-LVDP is further decreased because the introduction of the DWC structure results in the reduction of one side stripper.The decrease in the capital costs for SC-LVDP and DWC-LVDP are 3.56% and 6.44%, respectively.However, the introduction of the side-stream also increases the flow rate of the stripping stream,thereby inducing a 1.86%increase in TAC compared with LVDP.Moreover, SC-LVDP and DWC-LVDP significantly affect the flow rate distributions of the four products instead of the sum of the product yields.

5.Conclusions

In this study, HTO hydroisomerization reaction was investigated in an experimental fixed-bed reactor to achieve optimum liquid HIF by analyzing the impact of operating conditions(volume space velocity, temperature, and hydrogen-oil ratio).A LVDP for the separation of HIF is then proposed and optimized to produce lube base oils.Furthermore, two novel designs of LVDP including SC-LVDP and DWC-LVDP are proposed by coupling two thermal intensified technologies including side-stream and DWC.The following findings are drawn from the experimental and numerical results.

(1) Hydrocarbon composition of HIF are dominated by the depth of the isomerization and cracking reaction of HTO.The decrease in the volume space velocity and increase in the hydrogen-oil ratio will strength the isomerization over cracking reaction.The increase in reaction temperature results in the enhancement of both the isomerization and cracking reaction.However, an excessively high reaction temperature may weaken the selectivity of the isomerization reaction; hence, the total amount of alkane, 1-ring naphthene, and 2-ring naphthene of HIF deceases.

(2) The difficulty of the optimization of LVDP mainly lies in the the optimization of the VDC, owing to its numerous design variables and strong coupling among variables.Furthermore,the separation performance of fraction in the VDC can be improved with optimizing the drawn stage and heat duty of intermediate refluxs.This is because heavy component in the vapor and liquid-vapor load distributions are affected significantly be the drawn stage and heat duty of intermediate refluxs.

(3) The distributions of the vapor and liquid loads in LVDP, SCLVDP,and DWC-LVDP show similar overall trends that travel along zigzag paths.Compared with conventional structure LVDP, SC-LVDP and DWC-LVDP exhibit significant energy efficiency.DWC-LVDP further decreases in capital cost due to the removal of the side stripper and narrows the overlap between the third lube oils and fourth lube oils.The flow rate distributions of the four lube cuts vary in LVDP, SCLVDP, and DWC-LVDP.

In conclusion, this study illustrates that novel designs of LVDP combined with thermal coupling intensified technologies offer significant energy efficiency and capital cost advantage.Moreover,the flow rate distributions of the four lube base oils vary in different configurations.The specified configuration design for LVDP in industrial implementation can be achieved with comprehensive consideration of energy efficiency, capital cost, flow rate distributions of products,installation difficulty,etc.In contrast to other HTO studies that only focus on reaction systems, this study comprehensively investigates the reaction system to further focus on the optimization of the LVDP.The proposed approach and the results presented provide an incentive for the industrial implementation of high-quality utilization of HTO through intensified LVDP.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was kindly funded by Shanghai Sailing Program(No.19YF1410800) and National Natural Science Foundation of China(No.21908056).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.04.035.

Nomenclature

BI branching index

DWC-LVDP dividing-wall column coupling into side-stream lubricant-type vacuum distillation process

HIF hydroisomerization fractions

HTO hydrocracking tail oil

ID diameter of the prefractionator m

LVDP lubricant-type vacuum distillation process

P-1 pump-around circuit of vacuum distillation column

P-2 first intermediate reflux of vacuum distillation column

P-3 second intermediate reflux of vacuum distillation column

QDcondenser duty of the prefractionator, MW

QFheat duty of vacuum heat furnace, MW

QRreboiler duty of the prefractionator, MW

Qsumsum ofQFandQR, MW

RR reflux ratio

S-1 first side stripper of vacuum distillation column

S-2 second side stripper of vacuum distillation column

S-3 third side stripper of vacuum distillation column

SC-LVDP side-stream coupling into lubricant-type vacuum distillation process

SV volume space velocity, h-1

Ttemperature, °C

VH2/Vcatalystvolumetric flow rate ratio of hydrogen and catalyst

VH2/Voilvolumetric flow rate ratio of hydrogen and oil

VDC vacuum distillation column

VI viscosity index